This invention relates to a ring-laser gyroscope and more particularly to a laser gyro of a triangular type that overcomes "mode lock".
Ring-type lasers employing electromagnetic traveling waves at the optical or near-optical frequencies in a clockwise and counterclockwise direction about a closed path in a principal plane have been used to sense rates of angular motion similar in function to the well-known electromechanical gyro. The principle of the ring-type laser gyro is well known in the art.
Various arrangements of reflective surfaces have been used in an effort to provide a simple, effective laser gyro. The basic operation of a laser gyro has been set forth in an article "Laser Gyro" by Joseph Killpatrick, IEEE Spectrum, pps 44-55, October 1967.
One of the basic problems encountered in a ring-type laser gyro is the fact that the two beams tend to mode-lock for very low frequencies, that is, the two waves interfere with each other and each assumes a frequency which is substantially identical to the other. Mode-locking has been overcome by placing a magneto-optic mirror at one corner of an equilateral triangle or by the introduction of a Faraday cell in the optical path that introduces a bias. These systems create other problems which have to be overcome. Different systems for overcoming bias using the Faraday cell have been set forth in U.S. Pat. Nos. 3,862,803 and 3,890,047. These patents use two laser gyros in the same system which present a complicated system. It has been determined that one of the problems involves the properties of the materials employed in the magneto-optic elements. In either system, one wants a high degree of differentiation between the counter-propagating beams, that is, a large magneto-optic effect, and as little attenuation possible. Further, the gyro operation must be stable during operation.